1. Field
The present invention relates to a method and devices that can be used therein for the trenchless laying of pipelines in the ground.
2. Discussion of Prior Art
In the past, numerous methods and devices have been developed for laying pipelines in the ground without using trenches in order to pass under sensitive areas on the surface of the land for which laying in an open pipe trench did not appear to be possible or advisable for technical, ecological, legal or economic reasons. This may be the case for example whenever heavy construction machinery cannot travel onto the surface in the laying area (for example moors, bodies of water) or where no authorization for construction work can be granted from an ecological viewpoint (for example in nature conservation areas) or where the use of conventional laying techniques would be too expensive (for example where laying depths are great and the level of the groundwater is high).
In the literature there are extensive works on the laying methods that have already been used and tried out (for example Stein, D., Grabenloser Leitungsbau [trenchless line construction], 2003 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin, ISBN 3-433-01778-6). These have found that it is best for the method to be divided up on the basis of controllability (controlled/uncontrolled methods), soil handling (soil displacement/soil removal), transport of spoil (mechanical, hydraulic) and the number of working steps (pilot drilling, expansion drilling, drawing-in or pushing-in operation). Further distinguishing features are, for example, the basic geometrical formation of the drilling axis (straight, curved) and the pipe materials to be laid by means of the respective methods (for example concrete, PE, cast iron, steel, etc.). Furthermore, the achievable drilling dimensions (length, diameter, volume) are already among the suitable criteria for assigning specific methods to the same or a different group of methods.
Special attention also has to be given to the suitability of the methods for specific types of soil (grain size, grain shape, cohesive constituents, resistances, etc.), since most methods can only be used in certain soils and with certain groundwater levels (dry, earth-damp, water-saturated) or do not work under certain groundwater levels. Furthermore, the methods may also be distinguished by the location of the starting or finishing point (shaft, excavation, surface of the land).
With regard to the method according to the invention, the prior art is best represented by the so-called pilot headings, microtunneling (microtunnel construction, controlled heading) and the controlled horizontal drilling technique (flush drilling method, horizontal directional drilling, HDD).
In the case of the pilot headings, the laying takes place in two or three working phases, a controlled pilot bore of a relatively small diameter always been created first and then, in a further step, this pilot bore being expanded to the final diameter and the product pipes being pushed or drawn in at the same time. In this case, the laying takes place from a starting shaft to a finishing shaft.
The drilling lengths which can be achieved by these methods are generally less than 100 m and the diameters of the pipes that can be laid approximately between 100 mm and 1000 mm. The drilling (and consequently the pipe laying) generally takes place in a straight line, i.e. controlling the pilot bore has the sole purpose of laying the pipe in as straight a line as possible (for example for gravity lines). Owing to the method, the pipe runs are fitted successively while the drilling is being carried out, or while the individual pipes are being laid (headings, possibly interim pipes or temporarily introduced pipes, product pipes). A further feature of these methods is that these methods are relatively sensitive to certain soil properties (displaceability, water level, etc.), so that for example they do not come into consideration for laying a relatively long, large-caliber steel pipeline or in rocky soil.
In the case of microtunneling (MT), a controlled, sometimes curved, bore is created from a starting shaft or a starting excavation to a finishing shaft or a finishing excavation. It is characteristic of these methods that the pilot drilling, expansion drilling and the operation of pushing in the pipes are performed in a single working step. This combined working step is carried out in principle in a pushing or forcing manner from the starting shaft or the starting excavation, and the heading pipes, not connected to one another in a tension-resistant manner, correspond at the same time to the product pipes to be laid.
With this method, drilling lengths of up to 500 m and drill hole diameters of more than 2000 mm can be achieved. In addition, microtunneling can be used in virtually all types of soil (loose or solid rock) and in cases of virtually all groundwater levels with water pressures (up to 3 bar, possibly more).
Although the use of steel or PE pipes, for example, is possible in principle, it is unusual on account of the accompanying technical difficulties. PE pipes have, for example, a very low compressive strength (about 10 N/mm2) and consequently greatly restrict the possible laying range. Although steel pipes can be subjected to high axial loads, they must likewise be fitted pipe by pipe in the starting area and welded to one another in the process. For practical use, this has several disadvantages straightaway. On the one hand, the welding of large steel pipes is a time-consuming and complicated job (exact alignment and centering required), during which the actual drilling operation has to be interrupted. On the other hand, it is not possible before laying for the weld seams to be subjected to pressure testing, which is absolutely necessary for example when laying high-pressure gas lines or oil lines, since subsequent repair under the obstacle is virtually impossible.
Further disadvantages can be seen in the fact that steel pipe runs can only be controlled with great difficulty and it is accordingly necessary for the heading of such pipes to follow a generally straight laying plan, and the fact that the pipe casing (which is intended to protect the steel in the ground from corrosion) undergoes considerable loading during the heading, due to the direct contact with the wall of the drill hole, and is thereby damaged.
Finally, it should also be pointed out that, when steel or PE pipes that are designed as a pressure line are used, there is no possibility during heading to lubricate the outer casing of the pipes (for example with bentonite suspension), which leads to a significant increase in the casing friction occurring and, as a result, adversely influences the achievable drilling length.
The pipelines of relevance here (pressure pipelines of steel, PE, etc.) can consequently only be laid indirectly by means of microtunneling, in that conventionally a relatively large protective pipe string of normal heading pipes (concrete, polycrete, etc.) is laid, in which the actual product pipe run is then subsequently drawn or pushed. The disadvantages this procedure involves are obvious—creation of an actually too large drill hole diameter (for the protective pipes), costs for the protective pipes remaining in the ground, additional operation for the subsequent drawing-in of the product pipe run, costs caused by further equipment such as for example winches or the like.
In spite of all these disadvantages, the method described (microtunneling) represents the prior art for the laying of pressure pipelines in soils that are suitable for the controllable horizontal drilling technique described below (Tunnels & Tunneling International, March 2005, pages 18-21).
The third laying method to be mentioned in the context described here is the controllable horizontal drilling technique (abbreviated to “HDD” for horizontal directional drilling). With this three-phase method (pilot drilling, expansion drilling, drawing-in operation), only tension-resistant pipelines (for example of steel, PE or cast iron) can be laid. The geometrical laying capacities are superior to those for microtunneling in the case of the achievable length (>2000 m), but inferior in the case of the achievable pipe diameters (maximum about 1400 mm).
The greatest disadvantage of HDD is the great sensitivity to the ground conditions encountered in situ. In particular, gravelly, flinty or stony soils with less cohesive constituents almost always lead to problems if drill holes with a relatively large diameter (>800 mm) have to be created before the drawing-in operation.
The main reason for these difficulties is that, in the case of HDD, owing to the method, the drill hole is supported by the pumped drilling fluid alone (i.e. no interim pipes are fitted). In cases of unstable ground formations and large drill hole diameters, however, it is often not possible to achieve the required stability. Rather, the drill hole initially created collapses again in some regions after a certain time. As a result, it is virtually always impossible for a pipeline to be drawn in, and laying by means of HDD then fails (Tunnels & Tunneling International, March 2005, pages 18-21).
Additional difficulties for the HDD method, such as for example stones which jam between the wall of the drill hole and the pipe run while the pipe is being drawn in or damage said wall, and also the sometimes very high torques in cases of large drill hole diameters (for example in cases of drilling in solid rock), which have to be transmitted to the drilling head via the relatively thin drilling stem and not uncommonly lead to rupturing of the stem, are to be mentioned here only in passing. Similarly, the fact that, when using the HDD technique, owing to the method, the drill hole diameter has to be made about 1.3 to 1.5 times larger than the diameter of the product pipe run (otherwise there is the risk of seizing as a result of sloughing and sediment in the drill hole). This aspect is to be regarded as unfavorable from a technical and economic viewpoint.
To sum up the conclusions reached so far, it can be stated that none of the laying methods described is capable of laying a large-caliber, tension-resistant pipeline of great length reliably and effectively in difficult ground formations.